The OP has a 5DII and if I understand you correctly you appear to be suggesting that he can get good results with a 200mm or 300mm lens. I'm sorry if I have misunderstood, but that's crazy - unless they are stuffed. I have said that the minimum to get anything decent is 400mm on a crop camera.
That is what I use, and I have been shooting birds for a very long time. Mostly unsuccessfully it has to be said, because I have simply never had the reach and if I am going to sit in a hide for hours I don't want to come home to a load of post processing trying to pull three pixels of nothing into a recognisable image. So I intend to change that this year by hiring some suitable weaponry.
If I am recommending the OP to do anything, it would be to hire a something long and meaty before throwing money away on something that will not do the job it was purchased for.
Then all I can say is you are doing something very, very wrong...
Here are a few of my own examples for you.. EXIF should be embedded in them all should you wish to check.
One - Shot with 300mm on 1D 3 - slightly cropped
Two - both uncropped with 300mm on 1D 3
Three - Dunnock/robin uncropped with 70-200 on 1D 2
Four - Slightly cropped dunnock with 70-200 on 1D 2 I think
Five - Slight crops for framing 300-400mm on 1D 2
Six - Uncropped, 400mm on 1D 2
Seven - Slight crops (upto 25%) from 70-200 and 1D 2 - birds about 3ft away as I say in the thread..
... and there is plenty more where they came from... so please, do tell me that you NEED a minimum of 400mm on a 5D II... just think of the extra cropping the 20-odd megapixel 5DII has over the 8mp 1D 2 as well...
I said he could get results with the 100-400 easily.. for what he wants, garden/woodland birds and wetland centre type birds (ducks, geese, swans, gulls). If we do some simple maths, 300mm on my 1D is pretty much equivalent to 400mm on his 5D II. My 1D 3 has 10mp.. his 5DII has more than twice that. Aside the focus speed, drive speed etc.. he is in a fantastic position to get bird shots, so long as he is successful in bringing the birds in to range.
I never once said HE could get results with 200-300mm worth of reach, I said I had.. and if he were to concentrate on bringing the birds closer, as I did in my examples, then he too could possibly get similar results. I will reiterate for a 2nd time, that the 100-400mm lens would be an ideal choice for him. Giving him 400mm, not 200 or 300mm...
Your problem seems to be that whilst you are willing to sit in a hide for hours on end... you've never proactively tried to get the birds to come to you. They will. Just have a look at what Fracster and Sawman have done with their hides (there is a current thread at the moment). They dedicate lots of time, and effort... it isn't just about photography.
Ian has the ability to feed the birds in his garden on a daily basis, they'll get used to it, then he can introduce a hide/screen, let them get used to that.. then he can sit in/behind his construction.. let them get used to that, then finally he can introduce his camera..
Simples :shrug:
Ps. I think it's a bad idea to rent a big prime... as then when reality checks in and his budget (which I am assuming he has) limits him to a 100-400, he may wrongly feel it is inadequate and therefore give up on the idea all together.
If anything, he should hire a 100-400 (if not buy)... spend a weeks intense birding with it and see how he gets on. If he takes my tips on board he has no reason to fail... aside no photography skills... which I'm sure isn't an issue
